1. Field of the Invention
The present invention relates to a direct-current power source apparatus that supplies a load with a direct-current power discharged from an electric storage system.
2. Description of the Related Art
Conventional direct-current power source apparatuses use a capacitor or condenser with a relatively high capacitance to supply a direct-current power to a load such as a motor of an electric vehicle. See Japanese Utility Model Application Publication No. 03-104002 (JPU03-104002A) for example. Because the capacitor becomes unable to supply an electric power when charge falls short, conventional direct-current power source apparatuses additionally use a secondary battery to extend the time of use of a direct-current power. See Japanese Patent Application Publication No. 06-270695 (JP06-270695A), Japanese Patent Application Publication No. 2009-112122 (JP2009-112122A), Japanese Patent Application Publication No. 2004-266888 (JP2004-266888A), and Japanese Patent Application Publication No. 2010-4587 (JP2010-004587A), for example. In particular, Japanese Patent Application Publication No. 2009-112122 (JP2009-112122A) and Japanese Patent Application Publication No. 2010-4587 (JP2010-004587A) disclose that a lithium-ion capacitor is used as a high-capacitance capacitor to provide the maximum required electric power and to reduce the maximum electric power required for the secondary battery.
If the lithium-ion capacitor is overdischarged with its voltage falling below a lower-limit voltage inevitably determined by its characteristics, however, the lithium-ion capacitor may lose its original characteristics even if recharged. Therefore, it is difficult to fully utilize the lithium-ion capacitor in the conventional direct-current power source apparatus.
Meanwhile, the secondary battery tends to be degraded by quick charging and discharging, overdischarging, and overcharging compared to the lithium-ion capacitor, and thus may be degraded if controlled by a conventional control method. For example, in the configuration disclosed in Japanese Patent Application Publication No. 2004-266888 (JP2004-266888A), the secondary battery is connected in parallel with a main power source via a relay. Therefore, an unlimitedly large load may be connected to the secondary battery during charging and discharging. In the configuration disclosed in Japanese Patent Application Publication No. 2010-4587 (JP2010-004587A), the secondary battery is frequently used. In addition, an electric power is consumed at all times because a DC/DC converter is used at all times.